As per recommendation by loli frog, I tried out her build of DWM (I don't mind the pink color scheme so much because it's just a trial, but I'll change it to red tomorrow).
And I'm positively surprised.

On i3, I'd start with 600 MiB of RAM used, do what I always do, and end up with 3 to 5 GiB of RAM used up after closing everything down again (stuck in cache).
On DWM, I'd start with 400 MiB of RAM used, do the exact same things for the exact same length of time, close everything down, and end up with 600 MiB of RAM used.

So bye bye i3, and hello DWM!